Size effect of fish gelatin nanoparticles on the mechanical properties of collagen film based on its hierarchical structure

Fish gelatin nanoparticles (FGNPs) of different sizes (∼155–410 nm) were successfully obtained by adjusting the ratio of gelatin and glutaraldehyde (Gel/Glu ratio), and their possible generation mechanisms were revealed by Fourier transform infrared (FTIR) analysis and transmission electron microscopy images. The FGNPs were then used to fabricate collagen composite films. Compared to a pure collagen film, the tensile strength, elongation at break, and Young's modulus of composite films increased by 37.1%, 34.4%, and 79.2%, respectively, while the water vapor permeability decreased by 51.5%. Meanwhile, scanning electron microscopy images, FTIR, and X-ray diffraction analysis suggested that small-sized and uniformly distributed FGNPs maintained the original crystal structure and well-arranged super helical structure of collagen, while large-sized and cross-linked FGNPs destabilized the collagen structure. A Gel/Glu ratio of 12:4 afforded FGNPs with a size of ∼155 nm, which was the best collagen composite film-forming size. Moreover, the FGNPs could bind to collagen through weak but countless non-covalent bonds (hydrogen bonds and ionic bonds), thereby improving the mechanical properties of the films. Overall, the FGNPs improved the performances characteristic of collagen films by influencing their structure, including hydrogen bonding interaction, crystalline structure, and superhelix structure, which would advance the understanding of collagen-based films.